Laboratory exhaust hood

ABSTRACT

A laboratory exhaust hood is described including in combination a hood superstructure and a novel auxiliary air supply plenum. The auxiliary air supply plenum is of modular design suitable for use wherever an efficient conversion of a relatively high energy air stream of a given cross sectional area to a much lower energy laminar air stream of larger cross sectional area is desired. In particular, the auxiliary air supply is adapted for incorporation with a conventional laboratory exhaust hood, and has exterior vertical front, rear and end walls and a cover and interior baffles which define an air entry chamber, an air slot at the top of said chamber, an expansion chamber extending downward from the slot, an air balance chamber and an air supply outlet. Within the plenum, an air vector controller is positioned between the expansion chamber and the air balance chamber, and the final air balance means, including a back pressure plate, an air jet entrainment eliminator, and a shallow chamber therebetween, is positioned between the air balance chamber and the air outlet. The combination of the hood superstructure and the auxiliary air supply plenum includes a movable vertical closure sash and immediately above it, when the sash is closed, a sight-tight bypass. Beneath the closed sash is a horizontal air foil for the provision of a flow of air across the work surface of the hood, by the entrainment of room air in auxiliary air jets.

Uited States Patent 1191 Chamberlin et al.

[111 3,752,056 1 Aug. 14, 1973 LABORATORY EXHAUST HOOD [75] Inventors: Richard I. Chamberlin, Hanover; [57] ABSTRACT Joseph E. Leahy, West Quincy; Frederick vfles Not-wood a f A laboratory exhaust hood is described including in Mass combination a hood superstructure and a novel auxiliary air supply plenum. The auxiliary air supply plenum [73] Asslgnee sheldonand Company! is of modular design suitable for use wherever an effi- Muskegon, Mlchcient conversion of a relatively high energy air stream [22] Filed; Nov. 4, 7 of a given cross sectional area to a much lower energy laminar air stream of larger cross sectional area is del l PP N05 86,833 sired. In particular, the auxiliary air supply is adapted for incorporation with a conventional laboratory ex- 52 us. (:1. 98/115 LH, 98/121 haw hood, and has exterior vertical from, rear and s1 1 1111. C1. F23j 11/00 end Walls and a cover and interior baffles which define 58 Field of Search 98/115 LH, 101, 114, an air entry chamber. an air slot at the p of said 98/36, 37 121 chamber, an expansion chamber extending downward Y from the slot, an air balance chamber and an air supply [56] References Cied outlet. Within the plenum, an air vector controller is UNITED STATES PATENTS positioned between the expansion chamber and the air balance chamber, and the final air balance means, ingf 3 eluding a back pressure plate, an air jet entrainment 24320 l 11955 98/38 eliminator, and a shallow chamber therebetween, is po- 412 4/1963 H R sitioned between the air balance chamber and the air 1,390,347 9/1921 Elliston.... 98 114 x Outletl,652.128 12/1927 Hare 98/101 X The combination of the hood superstructure and the 3,000,292 9/ J 98/1 LH auxiliary air supply plenum includes a movable vertical I 11077 g 32 closure sash and immediately above it, when the sash 255: 981115 LH is closed, a sight-tight bypass. Beneath the closed sash 3408914 [1968 Bayem i n 98/115 L" is a horizontal air foil for the provision of a flow of air across the work surface of the hood, by the m my Examiner Meyer Peflin entrainment of room air in auxiliary air jets. Assistant Examiner-Ronald C. Capossela AttorneyRussell & Nields 22 Chums Dram;

Patented Aug. 14, 1973 5 Sheets-Sheet 1 RICHARD I. CHAMBERLIN JOSEPH E. LEAHY Patented Aug. 14, 1973 5 Sheets-Shoe L 2 unnn n vuurlvl A nu v1 vlu n n nvxr lllllllllllllIlIIIllllI llllllTllllllllllllllllllllllllHllllllllJlllIIH IIIIIIlllllllfllllllllllll Patented Aug. 14, 19.73

5 Sheets-Sheet 3 W F m 8 2 7 0 2 4 2 J f a w W, r ll-l. v v 2 9 3 3 3 3 U F w I'll! 1 8\ 5 u r W, m L xv p 4 K! H I M v 3 M MW n h I y 6 Patented Aug. 14, 1973 3,752,056

5 Sheets-Sheet 4 A x G 1 Tj Patented Aug. 14, 1973 3,752,056

5 Sheets-Sheet 5 LABORATORY EXHAUST noon BACKGROUND OF THE INVENTION This invention relates to an improved laboratory hood for use in air-conditioned laboratories in order to facilitate the handling of radioactive isotopes or other hazardous chemicals, to prevent the escape of dangerous toxic dust, vaporsor gases into the laboratory, and to provide safe, sanitary and healthy working conditions for laboratory personnel. In particular, the present laboratory exhaust hood represents a significant improvement in operation and safety over conventional hoods and specially designed hoods for hazardous use as described in the literature, for example in US. Pat.

Nos. 3,237,548 and 3,318,227, and in the references referred to and cited in said patents.

The accepted method employed for controlling laboratory air contamination and potential exposures of laboratory personnel to toxic, hazardous or radioactive materials isthe laboratory exhaust hood. The main purpose of a laboratory hood is to confine air contamination within the hood working area so that contaminant concentrations in the air in the workers breathing zone outside of the hood face are well below the threshold limit values. This is accomplished by exhausting air from the room and creating a flow of a clean air past the worker into the hood. In order for an air flow barrier to provide maximum effective worker protection from hazardous hood contamination, the air velocity must be adequate and uniform over the hood face with a velocity vector essentially perpendicular to the plane of the hood face opening. These are the fundamental concepts on which all acceptable laboratory hood design and performance must be predicated. For conventional laboratory hoods, therefore, our invention provides several features which overcome interferences with the face control velocity. Of prime importance is the reduction of interior hood turbulence, and secondly, the reduction of or prevention of excessive face velocities as the hood sash is lowered. This is achieved by provision of a special bypass for air to enter the hood as the sash is lowered with the bypass becoming operational before the velocity of entering air reaches a value at which undesirable turbulence occurs.

With the provision of greater numbers and increased sizes of laboratory hoods, a marlled increase in laboratory air changes has resulted and has imposed serious additional costs for both heated and in particular conditioned air. The need for a supplied or auxiliary air hood to reduce the cost has been recognized for inany years. However, prior to our invention, the designs have been such that the safety features which the hood was meant to provide have been seriously reduced or impaired. A basic criterion for a supply air design is that under normal operating conditions there be no compromise with the requirements mentioned for good conventional laboratory hoods.

In order to meet this criterion the auxiliary supply air must be provided entirely outside of the hood face, and in no way interfere with the control velocity required for containment of the contaminant within the hood. It should also be designed so that if the exhaust fan fails or exhaust air flow is reduced it would be at least no less safe than a conventional hood experiencing the same difficulty. Of course it should operate such that a sufficient quantity of air could be supplied, entrained, and exhausted to make its use economically feasible. In

the course of a complete review of the existing commercially available equipment, including those described in the above patents, it was found that the designs were such that potentially hazardous conditions could be expected with their use.

A general objective of the present invention is to convert an air supply stream to a uniform laminar flow pattern efficiently and within a minimum space. More specifically, an objective is to obtain an air bypass unit for laboratory hoods that assures completely adequate hood face air velocities under all conditions of hood face openings without excessive hood air velocities and hood air turbulence. A further objective is an air supply device (auxiliary air or makeup air unit for hoods or other needs) which can reduce the amount of air removed from laboratories or other areas (particularly, conditioned air).

Another objective is an auxiliary supply air device which can be attached to most if not all good laboratory exhaust hoods, to provide a uniform laminar (nonturbulent) flow of air downward and initially parallel to the front of the laboratory hood. If the amount of auxiliary air is percent or less of the total hood exhaust air quantity and the exhaust hood provides an essentially uniform exhaust at its open face, another objective is that at least percent of the auxiliary air be captured by the hood and enter the laboratory hood smoothly and without significant turbulence. In accordance with the present invention, the laboratory worker at the hood face is provided with smooth, clean, low velocity, noncontaminated air past his head and down in front of his work area, thus assuring a clean noncontaminated environment. Contamination origi nating within the hood is not only prevented from escaping-the hood into the room by the hood exhaust but it is also forced to remain in the hood by the positive supply air barrier. This double protection and clean air supply provide significantly improved protection from hood contamination losses caused by high room air convection and poor hood location. The remaining 25 percent, more or less, of air exhausted by the hood is taken from the room in which the hood is located. This room air is blended smoothly at the hood face resulting in a uniform air front into the hood producing minimum air turbulence with no reverse air flows.

A further object of the invention is to provide an air bypass device which both provides safety for the ope rator and permits air to bypass the hood face without either a significant pressure drop or excessive air velocities and turbulence in the hood. If a bypass of some type is not provided for hoods with vertical sliding sashes, the air velocity at the open face of the hood increases inversely proportionally to the size of the hood face opening as the sash is lowered. High face velocities and excessive turbulence are particularly undesirable for several reasons, including contamination of chemicals, equipment, and samples being analyzed, interference with burners and chemical reactions, and uncontrollable loss of toxic or radioactive materials. Safety of the operator is, of course, of paramount consideration. Therefore, as it relates to the bypass, the objective of this invention is to provide a bypass which ofi'ers protection for the operator without introducing any substantial, undesirable resistance to the free flow of air into the hood.

BRIEF DESCRIPTION AND ADVANTAGES OF THE INVENTION The present invention provides an auxiliary air supply plenum capable of providing air substantially vertically downwardly from its outlet in substantially uniform laminar flow, towards the face of a laboratory exhaust hood. Preferably, the auxiliary plenum is mounted above the hood face, with a bypass located therebetween. The auxiliary air supply plenum is operative to direct auxiliary air into the hood face, beneath a movable closure sash, when it is open, and above the sash through the bypass, as it is lowered and closed. It is a feature of the present invention, that the total volumetric flow of air into the hood superstructure, both room air and auxiliary air, remains substantially constant during movement of the hood sash between its open and closed positions. A further feature of importance in the air supply means of this invention is that the air supply is totally without prior contact with the interior of the hood.

The auxiliary air supply plenum is of modular design suitable for incorporation with conventional laboratory exhaust hoods, and in a preferred embodiment includes walls and baffies defining, in sequence following the flow of air from a supply conduit through the plenum, an air entry chamber, a vertical slot at the top of the entry chamber, an expansion chamber which extends downwardly and at right angles to the slot, an air balance chamber, and an air supply outlet. The device also includes an air vector controller between the expansion chamber and the air balance chamber and a back pressure plate and air jet entrainment eliminator between the air balance chamber and the outlet.

Among the features of the auxiliary air supply is its adaptation for efficient conversion of the relatively rapidly moving air stream, in the supply conduit, to a slower moving stream of substantially greater crosssectional area having a substantially uniform laminar flow free of non-uniform energy points within the stream, at the outlet. Another feature is the reversal of general fiow direction between the entry chamber and expansion chamber which permits accomplishment of the objective with a minimum of height for the plenum. Still another feature relates to the vertical slot between the entry and expansion chambers. The slot is dimensioned relative to the cross sectional area of the entry chamber to cause a general distribution of the air to the inlet end of the expansion chamber. A feature relating to the expansion chamber itself is that its walls are gauged to provide a generally uniform adiabatic expansion for the air. In addition, a feature of the combined elements downstream of the expansion chamber is the provision of elements which in combination provide a slight but important back-pressure, coupled with a greatly multiplied, highly dispersed closely spaced substantially uniformly resistant air paths. The end result is the virtual elimination of air jets and air jet entrainment (air picked up by the jets and entrained with them), such that the air flow from the auxiliary supply is substantially laminar and free of non-uniform energy spots.

The laboratory exhaust hood combination of the present invention includes a hood superstructure having a horizontal work surface, rear, side and front walls, and an open vertical face together with conventional air balancing baffles. A movable, vertical closure sash is located on suitable tracks immediately inside the front wall. A sight-tight bypass is provided immediately above the open face, in such a position that as the movable sash is lowered to close the face the bypass becomes open, and when the hood is completely closed the bypass is fully open and receives substantially all of the auxiliary air. In either case the auxiliary air exits the air supply plenum substantially vertically downwardly in substantially uniform laminar flow, whether it is directed to the open face, to the bypass, or partly to both.

Among the advantages of the present bypass, as the sash is lowered to close the hood, the maximum velocity provided to the face is 50-80 percent of that created by conventional laboratory hood bypass arrangements, such as those referred to in the above patents. Moreover, in the present device the operator is protected since the bypass is sight proof and accordingly offers a significantsafety barrier between the inside of the hood and the external work areas with simultaneously impairing the uniformity of air flow.

The use of auxiliary air supply plenum and bypass in combination of this invention in a laboratory exhaust hood provides excellent performance. The auxiliary air is substantially completely captured by the hood and the air velocity profile at the hood face is substantially uniform for all sash positions. The sash never extends into the air supply, and consequently the auxiliary air cannot be contaminated by either the interior of the superstructure or the sash.

Beneath the closed sash, above the front edge of the hood work surface, is a horizontal air foil of novel construction. The air foil is provided with auxiliary air from the supply plenum by way of a by-pass conduit. The air is blown under the air foil toward the work surface where it entrains room air, by means of a horizontal line of air jet orifices. Preferably at least -80 percent of the air passing over the work surface from under the air foil is entrained room air, the remainder being the auxiliary air.

The air inlet to the auxiliary supply plenum may be at the top, side, back or front, making it readily adaptable to any supply duct configuration. As the air passes through the plenum it is efficiently and with a minimum of height reduced in energy and distributed in a manner which provides uniform, nonturbulent nonpulsating air to the hood.

Among the additional features of the auxiliary air supply plenum are: (a) all of the air is provided outside the hood; (b) at least percent of the air supply is efficiently captured by the hood, due to the uniform laminar flow of air; (c) a steady supply of low velocity, clean air in the zone of the operator is provided, assuring clean air free from hazardous materials; (d) heating units may be readily incorporated into the plenum without changing the size or the shape or affecting uniform air delivery; (e) the plenum may be modular and capable of ready installation on any good laboratory hood without disturbance to exhaust ducts or other possibly contaminated areas of the hood; and (f) the unit may be readily adapted to receive supply air from the rear, front or either side, as well as from the top as specifically described herein.

The specific bypass employed in the combination of this invention also obtains several combined advantages: (a) there is less air turbulence in the hood than with comparable bypass systems; (b) the bypass controls the increase in face velocity; (c) the pressure loss through the bypass is extremely low, insuring substantially constant air flow through the hood, even with the sash completely closed; and (d) because the preferred bypass is contructed of V-shaped louver vanes providing a deflected air path, an effective safety barrier against the direct flight of material is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevation in perspective of a laboratory exhaust hood combination employing the auxiliary air supply plenum and preferred bypass of the present invention shown in a typical fume hood construction;

FIG. 2 is an enlarged cross sectional view in elevation of the preferred auxiliary air supply plenum;

FIG. 3 is a cross section view of the plenum illustrated in FIG. 2, takenalong line 3-3 thereof;

FIG. 4 is an enlarged elevation view in cross section of the V-shaped louver vanes which comprise a preferred bypass;

FIG. 5 is an enlarged cross-sectional view of the bottom air foil, showing the entrainment of room air by the auxiliary air jets;

FIG. 6 is a view similar to FIG. 2, showing the air flow through the air supply plenum during operation;

FIG. 7 is a view similar to FIG. 3, showing the air flow through the air supply plenum during operation;

FIG. 8 is a cross sectional view in side elevation of an exhaust hood employing the plenum of FIGS. 2-3 and the bypass of FIG. 4 in combination with a hood superstructure, showing the direction of air flow with the sash in fully open position, through the open face and the bottom air foil, taken in section at the face center;

FIG. 9 is a view similar to that of FIG. 8, showing the air flow through both the bypass and the bottom air foil, with the sash in fully closed position; and

FIG. 10 is a view similar to that of FIGS. 8 and 9, showing the air flow through the bypass, the partially opened face and the bottom air foil with the sash partly closed.

DETAILED DESCRIPTION OF THE INVENTION The present invention is broadly directed to apparatus for converting rapidly moving air to slower moving air, including in combination means for receiving the air stream, means for expanding the air unifonnly across its cross-section and means for distributing the air uniformly from the receiving means to the expansion means. The distributing means preferably is a elongated slot of a sufficiently small cross-section to promote a spreading of the air along the slotlength.

The preferred embodiment of the present invention includes an auxiliary air supply means, suitably of modular form, of novel construction and operating capabilities. The auxiliary air supply means is preferably employed in combination with a laboratory exhaust hood superstructure, and particularly adapted to supply auxiliary air to the hood. The invention also includes in that combination a novel bypass construction, which permits both the auxiliary and the room air to pass through the bypass without risk of contamination because of prior contact with the hood interior, and substantially uniform exhaust flow even as the hood sash is lowered into closed position, together with shielding against the direct flight of particles from within the hood.

The auxiliary air supply plenum is substantially rectangular in construction, having an air inlet at vor near the top and an auxiliary air outlet at the bottom. The auxiliary air supplied from the'outlet is substantially uniform and laminar, and flows substantially vertically downwardly from the plenum.

At the top of the plenum is an air entry chamber; along the top of one side of which is a horizontally extending vertical slot. Extending downwardly from the slot, and at substantially right angles to it is an expansion chamber. Preferably, the relationship between the air inlet, entry chamber, slot and expansion chamber is such that: the inlet is spaced away from the slot and ex pansion chamber; the vertical, cross-sectional area of the entry chamber is at least twice the vertical crosssectional area of the slot; said slot area is less than about 1.5 times the horizontal cross-sectional area of the inlet to the entry chamber; and the air changes or reverses direction when passing through the entry chamber and expansion chamber. The expansion chamber is preferably at an expansion angle of about 9, which serves to expand the air received from the vertical slot adiabatically into an air balance chamber at about the middle of the plenum. An angle of between 7 and 12 will provide adiabatic expansion. An air vector controller, suitably of egg-crate" type construction, is situated at the end of the expansion chamber, and, together with a slight back pressure, assists in directing the expanded air into vertical and uniform flow. Preferably, the pockets of the air vector controller are rectangular or square, and at least as deep as wide.

The air balance chamber extends substantially across the entire width and length of the air supply plenum, and is hence considerably larger in cross-section than the expansion chamber, which in cross-section occupies between about 10 and 25 percent of the plenum. Consequently, the air flow rate is considerably reduced upon arrival at the air balance chamber, thus assisting attainment of uniform, laminar flow.

Beneath the air balance chamber is a final air balance means, which includes a perforated pressure plate, an air jet entrainment eliminator, with a shallow chamber therebetween. Air flow through the pressure plate is substantially laminar, which further assists in the attainment of uniform laminar flow in a vertical direction downwardly.

Preferably, the orientation and dimension of the ex pansion chamber are such that the cross-sectional area of the expansion chamber projected upon the perforated plate is at least l.4 times the free area of the plate perforations.

A unique feature of this combination is its ability to eliminate localized non-uniformity of air energy across the auxiliary air supply in a minimum of vertical space. This is assisted first by changing or reversing the air flow direction between the air entry chamber and the expansion chamber through the vertical slot. Thereafter, the vector controller, balance chamber, pressure plate, shallow chamber and air jet entrainment eliminator cumulatively contribute to the desired result.

Preferably the air supply plenum includes immediately beneath the air outlet, parabolic profile vanes, one on each end wall, which serve to force the air somewhat inwardly from the two edges. Preferably the air flow rate as it enters the open face of the hood superstructure, is such that a plot of the air rate from edge to edge across the open face would assume the curvature of a shallow parabola, having a maximum at or near the center of the face, and substantially equal minimums at each end. Such a profile is desirable in order to avoid turbulence at the sides of the face and work area, and to avoid loss of auxiliary air into the laboratory. In the auxiliary air supply plenum, with the parabolic profile vanes, it is possible under normal operating conditions for the hood face to recover at least 95 percent of the air exiting the plenum outlet.

The present auxiliary air supply plenum in modular form is readily attached to laboratory exhaust hood superstructures, above and in front of the open face. Thus it is possible to obtain the benefits of the present invention, at least in part, without the expense of an entirely new laboratory exhaust hood, including superstructure. Preferably, however, the present auxiliary supply plenum is used in combination with the hood superstructure herein described and the novel bypass construction of the present invention. While the preferred mode of operation includes directing the air vertically downward and special benefits are derived therefrom, certain advantages of the auxiliary supply of laminar air flow can be obtained regardless of the direction of air travel. Therefore, as it relates to the auxiliary air supply per se, it is a feature of the auxiliary air supply that it can be employed without limitation to any special orientation of the air flow direction.

The present bypass is located immediately above and in front of the open face, in place of a portion of the front wall which exists at that location in conventional exhaust hoods. As the sash is lowered from its fully open position to its closed position, the bypass becomes open to the interior of the hood superstructure and part of the auxiliary air is bypassed in that direction above the sash. As the sash is lowered even further and almost closed, more and more of the auxiliary air passes through the bypass. The construction of the auxiliary air supply plenum and bypass, and their location in the present invention is such as to minimize the pressure drop of the air when the sash is completely closed from the auxiliary supply to the interior of the hood superstructure. The result, therefore, is to maximize the flow of air to the exhaust when the sash is closed, such that the rate of air flow is substantially constant whether the sash is open, partly open or closed. One feature of the bypass is that although it provides a sight-tight labryinth, it does not require the air flow to change direction at any one point sharply enough to impair the kaminar air flow or to introduce local points of uneven energy in the air stream. Still another feature is that the bypass directs the air upwardly into the same path as that followed by air coming under the sash.

The invention will be better understood by reference to the attached drawings, wherein like reference numerals indicate like or corresponding supply 12. Located immediately belowthe auxiary r supply 12 illustrate preferred embodiments of the invention, an which demonstrate its advantages.

In FIG. 1 there is shown a laboratory exhaust hood 10, including a hood superstructure l l and an auxiliary air supply 12. Located immediately below the auxiliary air supply 12 is bypass 13. Within the superstructure l l is work surface 14. The superstructure 11 includes rear wall 15, side walls 17 and 16, front wall 18, open face 19, closure sash 20 and sash window 21, rear baffle 22 and exhaust outlet 23. The auxiliary air supply 12 includes auxiliary air inlet 24, triangular side baffles 25 and 26, and adjustable vane 27.

FIGS. 2 and 3 show the details of auxiliary air supply 12, including the above mentioned auxiliary air inlet 24, triangular baffles 25 and 26, and adjustable vane 27. In addition, the auxiliary air supply 12 includes a front wall 28, rear wall 29, end walls 30 and 31, and cover 32. Immediately within the air inlet 24 is air entry chamber 33, bounded by rear wall 29, and baffles 34 and 35, baffle 35 including baffle reinforcement 35a. Air entering inlet 24 surges into chamber 33 and reverses direction moving upwardly towards slot 36, a substantially rectangular slot oriented in a vertical plane bounded by the top of baffle 35, cover 32, and end walls 30 and 31. Preferably, the air inlet 24 is as large as practical, and it may be as long as the chamber 33, although it should not extend over the top edge of baffle 35. In order to maintain a low sound level, the minimum area of the inlet 24 is one-third the width of plenum 12, times 0.24 times its length. The orientation shown in the figures, with the air inlet 24 and entry chamber 33 adjacent rear wall 29, may of course be reversed to locate these elements in the front of plenum 12, adjacent front wall 28 (as shown in the alternative embodiment shown in FIG. 9). Also, air inlet 24 may be located in any position desired in the top 32, or rear wall 29 (as shown in the alternative embodiment shown in FIG. 10), to provide for air entry into chamber 33, provided that the inlet 24 does not extend over the edge of baffle 35 forming slot 36. In this respect, the present air supply provides the unique advantage of being able to be connected with an air conduit at the top, side, front, or rear. Preferably, air inlet 24 is centered in the top of the air supply at either the rear edge, as shown in FIGS. 2 and 3, or front edge thereof. The cross-sectional area of the slot 36 has an important relationship to the vertical cross section of entry chamber 33, such that the air tends to spread along the full length of the slot rather than channelling through the slot in one place. The chamber 33 is at least twice as high as slot 36, and preferably at least three times as high. The air passes through slot 36 substantially horizontally and rapidly across the length of the air supply 12 into baffle 37 which, in combination with baffle 35, forms expansion chamber 38, extending downwardly from and substantially perpendicular to slot 36. Each of the baffles 35 and 37 are angled at about 4.5, to give expansion chamber 38 an expansion angle of about 9, which is best for a slot 36 width of about 1.7 inches. It has been determined that a total angle between about 7 and l2 is adequate for achieving the desired adiabatic expansion. Preferably, a dividing baffle 39 is centered in expansion chamber 38 in order to fonn two compartments, which is particularly advantageous for exceptionally long expansion chambers. Preferably the horizontal opening of expansion chamber 38, at the top of battle 35, is the same length and width as vertical slot 36, or in any event the area thereof is 0.8-1.2 that of slot 36. In the embodiment shown, the expansion chamber 38 is as long as the height of entry chamber 33, less the height of slot 36. This arrangement permits the reversal of air flow from the entry chamber 33 to expansion chamber 38 within a minimum height. FIG. 2 shows this arrangement with the elements to scale, in which the entry chamber is 8-inches high, slot 36 is 1% inches high, and expansion chamber 38 is 6% inches high, 1% inches wide at the .top and 2% inches wide at the bottom. At these dimensions, the length of chamber 38 is about at its minimum, although it may be longer if desired, with appropriate rearrangement or lengthening of the elements. At the end of expansion chamber 38 is air vector controller 40, preferably constructed of one-half inch cubed plastic louvers, sometimes described as egg crates. The depth of the louver units is desirably at least equal to their width.

Air vector controller 40 is attached to the bottom of baffle 34 and baffle 41, which combination forms the top of air balance chamber 42. Since air balance chamber 42 extends substantially across the entire cross section of the air supply plenum 12, the air which is rapidly passed through expansion chamber 38 quickly slows down upon exiting through air vector controller 40, thereby achieving considerable lowering in velocity, and a laminar air flow is achieved within a minimum of height. Preferably, a desirable relationship between the expansion chamber angle and dimensions and the free area and location of the perforated plate 44 is maintained, such that the following relationship exists:

kA,w s 2 (1+ h) tan (/2); wherein k is a constant,

A, is the fraction of free area in perforated plate 44,

w is the width of plenum 12, or plate 44,

s is the width of the entry slot of chamber 38,

I is the length of expansion chamber 38,

h is the height of balance chamber 42, and

0 is the expansion angle of chamber 38.

Preferably, the constant k is at least 1:4; A, is about 0.19 0.25; w is about 12 inches, standard; s is about 1.4 1.8 inches; 1 is about 6-8 inches; h is at least 0.5 times w; and 0 is 7-l 2, being inversely related to s. In the illustrated embodiment; A 0.2, w 12 inches s 1.625 inches, 1 6.375 inches, h 6 inches, 0 9, and k l .5. The value of preferably should be such that the width of the outlet of expansion chamber 38 is at least 0.9 (A (w). While h is preferably at least 0.5 the value w), it may be less if reduction in height of plenum 12 is a major consideration. The value of s may be less than 1.5 inches, at a sacrifice of increased plenum pressure and noise; ifs is greater than 1.8, poor air spreading may result, requring an increase to l or a decrease in 6 below 7.

Beneath air balance chamber 42 is a final air balance means 43, including back pressure plate 44 and air jet entrainment eliminator 45, defining a shallow chamber 46 therebetween. Pressure plate 44 may suitably be constructed of a perforated Iii-inch board hav lng /4-inch holes on r-inch centers. Passage of the air through the holes of pressure plate 44 is substantially uniform and laminar. In the illustrated plenum, for la-inch holes in plate 44 a Reynolds number of 2140 is obtained, and for 9/32-inch holes, the value is 1,900, and hence laminar flow through plate 44 is achieved. The depth of shallow chamber 46 is controlled by the diameter of the perforations in plate 44, such that the air jets leaving the perforations preferably entrain the air adjacent the closed portions of plate 44before arrival at the top of air jet entrainment eliminator 45. Air exiting the holes of pressure plate 44 flows uniformly and without pulsation and expands and fills shallow chamber 46, bounded downstream by air jet entrainment eliminator 45, suitably of low resistant, all glass fiber construction. This air jet entrainment eliminator blends any minor non-uniformities or pulsating flow into a smooth air flow which is discharged from the unit truly laminar, moving in a substantially, non-pulsating, vertically, downward direction. The final air balance means 43 may be constructed of radiator type, honeycomb fins of high open area and very low closed area, of sufficient depth to provide the same back pressure as the device shown. In such an embodiment the need for an air jet entrainment eliminator is obviated. Such an embodiment may be preferred where, for example, it is desired to heat the air stream by means of such a radiator.

The many steps involved in the present air supply plenum in balancing the air and recovering air energies not only provide better air balance and uniformity than possible with conventional units, but this is also achieved at a low air noise level.

Beneath air supply outlet 47 are curved profile vanes 48 and 49, which serve to reduce air velocity at the ends of the outlet and increase the velocity at the center. As a result the flow-rate profile into the open face of the exhaust hood superstructure assumes a shallow parabola, having a maximum at or near the center, and substantially equal minimums at each side. This type of air profile is desirable since it minimizes losses of air supply to the laboratory, and reduces the likelihood of turbulence at the sides of the hood face. The profile vanes are suitably constructed from 50 percent freearea screens. At the bottom of the frontwall 28 of the air supply 12, there is an adjustable vane 27 hinged to front wall 28, and secured by adjustable engagement means 50. In general, the adjustable vane 27 will be pointed at the lower closure point of the sash, but will be adjusted inwardly or outwardly, by engagement means 50, depending upon whether the auxiliary air is respectively hotter or colder than the room air. It will also be adjusted inwardly if there is excessive turbulence in the room air.

The bypass device useful in the combination of the present invention is shown in detail in FIG. 4 and in the present combination in FIGS. 1, 8, 9 and 10. Bypass l3 is constructed of V-shaped angled louver vanes 51,

which define V-shaped labyrinth passages 52. As air passes through bypass 13, an upward direction upon the air is induced by the vanes 51, but with minimum pressure drop. Bypass 13 also provides a sight-tight closure above the open face 19. Our tests have shown that the vanes may define an angle of without introducing a significant pressure drop or disturbing the laminar flow of the air, and that a sight-tight labyrinth is provided by employing 2 inches wide, 20-gauge metal strips space one-half inch on centers having 7/16 inch clearance therebetween. This angle only re-directs the air flow through a 60 angle. An angle of 50-70 would be feasible, with corresponding increase or decrease to the number of vanes 51. It is possible to achieve virtually equivalent bypass air flow by turning bypass l3 upside down, although such an orientation would be incapable of holding any possible liquid splashes, as with the illustrated bypass l3, and hence is less preferable.

FIG. 5 illustrates the details of a preferred horizontal air foil providing auxiliary and entrained room air' to the work surface 14. The foil is comprised of horizontal plate 59, slanted plate 60 and end plate 61. Air jets, shown as solid arrows in FIG. 5, emit from entrainment air conduit 62 through air orifices 63. Conduit 62 is connected via air inlet 64 and suitable hosing, not shown, to outlet 56 (FIG. 2). The orifices 63 are positioned 4 inches in front of edge 65 of work surface 14, and, at a flow rate of about 1.33 cfrn per linear foot of conduit 62, are capable of entraining up to 9.5 times as much room air. In order that at least 70 80 percent of the bottom air flow be room air, orifices 63 are positioned at least 3.5 inches in front of front edge 65 of work surface 14. Preferably, orifices 63 are /64-inch holes on ra-inch centers across the length of conduit 62, and conduit 62 has a cross-sectional area of at least 0.8 square inches. The initial velocity of the air jets in the illustrated embodiment is about 2,000 fpm. The residual velocity of the air jets at front edge 65 is about 180 fpm in the illustrated foil 55, which offers a decided improvement over prior foil jets, having a residual velocity of about I000 fpm, reflected in considerably decreased turbulence above the work surface 14. The shape of conduit 62 is not critical, as it may be round, trapezoidal, square or rectangular; similarly, the distance of orifices 63 from slanted plate 60 is not critical, but preferably they are centered. As can be seen by the schematic arrows showing air flow, the air jets (solid) entrain room air (dashed), and direct the resulting stream of air up to the opening between plate 59 and work surface 14. Because the orifices 63 are spaced a substantial distance from the front edge 65, the room air is completely entrained and constitutes 90 percent or more of the total air flow to the work surface 14.

FIGS. 6 and 7 show air flow through plenum l2 during operation. Air enters inlet 24 very rapidly and is dispersed in chamber 33 across its width and length (see dashed arrows, showing air movement behind baffle 35, in FIG. 7). The air reverses direction and passes up over baffle 35 through slot 36, and enters expansion chamber 38. Upon leaving chamber 38, through air vector controller 40, the air surges into balance chamber 42, with some entrainment and turbulence, but with considerable decrease in velocity. Finally, the air passes through plate 44 and air jet entrainment eliminator 45, and emerges from outlet 47 in uniform, laminar flow, and passes by profile vanes 48 and 49, and adjustable vane 27, where the uniform stream is directed towards the hood face.

FIGS. 8, 9, and illustrate air flow of both the auxiliary air and room air into the hood superstructure with the sash in various positions. Closure sash 20 moves vertically up and down on closure tracks 53. In these figures, the flow of supply air is shown by solid arrows and the flow of room air is shown by dashed arrows.

In normal operation as shown in FIG. 8, sash 20 is fully opened up to the top of tracks 53. Auxiliary air exits supply outlet 47 in a substantially vertical downward direction, and passes through open face 19 of superstructure 11 into the hood interior above work surface 14. Similarly, room air passes through open face 19. The air passes into the hood uniformly and with minimum turbulence guided by side edges 54 and bottom air foil 55. To a lesser extent, air, both from the room and from auxiliary air provided through bypass conduit 56 through inlet 64 and air jet orifices 63, enters under air foil 55, with the jets serving the function of entraining and assisting the movement of air into the hood at that point. The air within superstructure 11 passes around rear baffle 22 and baffles 57 and 58, leaving through exhaust outlet 23. When the sash 20 is completely closed, as shown in FIG. 9, air flow is principally through bypass 13, with a small amount of air still passing under the closed sash 20 and bottom air foil 55. As shown by the arrows both auxiliary air and room air pass through bypass 13 into the interior of superstructure ll, being given an upward thrust by vanes 51 during passage through V-shaped passages 52. FIG. 10 illustrates the air paths during the process of closing sash 20, while the sash is partly closed to an extent sufficient to open bypass 13. It can be seen that a portion of the auxiliary air passes through the upper part of bypass 13 into the hood superstructure interior, while the bulk of the auxiliary air still passes through the face 19 under sash 20. At this point the room air mixes with the auxiliary air and enters the hood in increasing proportion toward the lower end of the face. At the base, air from both the auxiliary supply and from the room passes under air foil 55 into the interior.

In the embodiment illustrated in the figures, the auxiliary supply is capable of delivering up to percent of the exhaust air, at face velocities of 50-150 fpm, or more. The flow is uniform and laminar, and meets the most exacting design specifications, and the objectives stated herein.

It will now be apparent that various modifications can be made to the preferred embodiment without departing from the spirit of the invention in its various aspects. For instance, certain of the advantages of laminar flow provided by the auxiliary air supply can also be employed in other uses. Therefore, in its broadest usage the auxiliary air supply per se should not be limited to direction of air flow. The same applies to the bypass which provides certain advantages whether positioned above, below or to the sides.

In addition the combined air jet entrainment elimination function of the elements downstream of the expansion chamber does not require the precise components shown, although they accomplish the objective in the most efficient manner. For instance, in an apparatus designed to regulate the temperature of the auxiliary air, a fin tube heat exchanger with extended honeycomb air passages can be employed, and provided the walls of the passages are sufficiently long to provide a significant back-pressure and the air outlet openings on the downstream side are not separated by large closed areas, such a heat exchanger can, by itself, provide air jet entrainment elimination, thereby embodying the functions of both the pressure plate and entrainment eliminator. Moreover, it is possible to insert heating elements, such as calrod units, in the air balance chamber, or in the air entry chamber, in the supply plenum.

Thus, it is not our intention to confine the invention to the precise form herein shown but to limit it only in terms of the invention, as particularly and distinctly pointed out in the appended claims.

We claim:

1. Apparatus for converting a rapidly moving air stream to slower air having substantially laminar flow free of localized non-uniform energy comprising: receiving means comprising an entry chamber for receiving said air stream; means including two diverging walls for expanding the flow of said stream substantially uniformly across the cross-section of its flow; and means for distributing the flow of said stream substantially uniformly from said receiving means to said expansion means; said distributing means comprising an elongated slot having a cross-sectional area which is sufficiently smaller than the cross-sectional area of said entry chamber transverse to the flow of said air to promote a spreading of the flow of said air into such slot along its length.

2'. Apparatus for converting a rapidly moving air stream to slower air having substantially laminar flow free of localized non-uniform energy, comprising: receiving means comprising an entry chamber for receiving said air streams; means including two diverging walls for expanding the flow of said stream substantially uniformly across the cross-section of its flow; and means comprising a passage between said receiving means and said expansion means for distributing the flow of said stream substantially uniformly from said receiving means to said expansion means, wherein said entry chamber and air expansion means are adjacent, share one of said walls as a common wall, and are constructed and arranged to reverse the direction of flow of said stream.

3. Apparatus for converting a rapidly moving air stream to slower moving air having substantially laminar free flow of localized non-uniform energy, comprising means for receiving said air stream; means including two diverging walls for expanding flow of said stream substantially uniformly across the cross section of its flow; means comprising a passage between said receiving means and said expansion means for distributing the flow of said stream substantially uniformly from said receiving means to said expansion means; and means for air jet entrainment being downstream of said expansion means and including a multiplicity of closely spaced widely distributed air flow openings, together with means operatively associated with said openings upstream thereof for effecting a slight backpressure.

4. The apparatus of claim 3 further characterized by an air balance chamber between said air expansion means and said air jet entrainment elimination means.

5. An auxiliary air supply plenum of modular design suitable for incorporation with a conventional laboratory exhaust hood, which hood is provided with an air exhaust means, an interior work space, a front face, and a movable closure sash in said face for supplying a substantially uniform laminar flow of auxiliary air to said exhaust hood, said auxiliary air supply comprising:

exterior front, rear and end walls, a cover and interior baffles defining (l) an air entry chamber, (2) a slot along the upper edge of one wall of said entry chamber, (3) an expansion chamber extending vertically downwardly from said slot toward the interior of the'plenum and (4) an air supply outlet downstream of said expansion chamber; wherein said chambers, slot and outlet extend substantially the length of said plenum and said outlet extends substantially across the width of said plenum.

6. The air supply plenum of claim 5 wherein the width of said slot is no more than one-half the height of said entry chamber, and said entry chamber and said expansion chamber share a common wall having said slot at the top thereof.

7. The air supply plenum of claim 5, wherein an air balance chamber and final air balance means are provided between said expansion chamber and said outlet, said air balance means including (1) a back pressure plate perforated with laminar flow holes substantially uniformly across its width and length, (2) an air jet entrainment eliminator, and (3) a shallow chamber between said plate and said entrainment eliminator.

8. The air supply plenum of claim 7, wherein the dimensions and orientation of the elements thereof are such that:

kA,w s 2(I+h) tan (0/2) wherein k is atleast 1.4,

A, is the free area of said plate,

w is the width of said plenum,

s, the width of said slot, is l.4-l.8 inches,

I is the length of said expansion chamber,

h is the height of said balance chamber, and

0, the expansion angle of said expansion chamber, is

9. The air supply plenum of claim 7, wherein said air supply outlet is provided on each of its end walls with curved profile vanes, whereby air exiting said plenum tends to assume a flow rate profile parabolic in shape along the length of the outlet, and the portion of the front wall defining the front of the air supply outlet is an adjustable vane hinged at the top and adjustable from the vertical toward the rear wall of the outlet, whereby the auxiliary air velocity may be varied.

10. The air supply plenum of claim 7 wherein said back pressure plate is a fin-tube heat exchanger, whereby auxiliary air may be heated between said air balance chamber and said shallow chamber without affecting the substantially uniform laminar flow of air from said air supply outlet.

11. A laboratory exhaust hood, having an open face, a bypass incorporated in said hood above said open face, a movable closure sash adapted to move from an open position, in which it closes said bypass and opens said face, to a closed position in which it opens said bypass and closes said open face, and means to maintain the total volumetric flow of air into said hood substantially constant during movement of said movable closure sash from its open position to its closed position comprising a plurality of substantially parallel walls arranged in said bypass sequentially in angular relation to provide a sight-tight labyrinth, wherein the angular relation of said walls forming said labyrinth to change direction within the range of substantially 50 to at any one corner.

12. The laboratory exhaust hood of claim 1 1, wherein said hood has a front wall inwhich said open face is located, and said hood is also provided with an auxiliary air supply plenum located above said open face, and wherein said bypass is constructed of a series of angled louver vanes extending horizontally across the length of said front wall from the bottom of said plenum to the top of said open face.

13. The laboratory exhaust hood bypass of claim 12, wherein said louver vanes are angled to form a vertical series of V-shaped passages and adapted to induce an upward direction upon air traversing said bypass.

14. A laboratory exhaust hood including in combination a hood superstructure having a vertical, rectangular face and a movable vertical closure sash, and an auxiliary air supply plenum, comprising: means within said auxiliary air supply plenum to provide auxiliaryair substantially vertically downwardly'from said plenum toward said open face of said hood superstructure and in substantially uniform laminar flow; bypass means operative upon the closing of said movable, vertical closure sash to direct at least a portion of said auxiliary air into said hood super-structure by passage through a sight-tight by-pass above said sash; and means to maintain the total volumetric flow of air into said hood superstructure substantially constant during movement of said movable closure between its open and closed positions.

15. The laboratory exhaust hood combination of claim 14, wherein further means are provided to control the maximum face velocity of the air entering the hood superstructure through said rectangular open face during the movement of said movable closure from its open position to its closed position between 2-3 times that of normal operation, with a fully opened sash.

16. A laboratory exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising:

a. a horizontal work surface, a rear wall, side walls,

and a front wall including an open vertical face immediately in front of said work surface; and

b. auxiliary air supply means including means to provide auxiliary air from a point above said open vertical face downwardly to substantially all parts of said open face in a substantially uniform laminar flow: wherein said auxiliary air supply means comprises an entry chamber, a slot in one wall of said entry chamber having a width no more than one-half the dimension of said entry chamber parallel to said slot, and an expansion chamber extending from said slot.

17. A laboratory exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising:

a. a horizontal work surface, a rear wall, side walls,

and a front wall including an open vertical face immediately in front of said work surface; and

b. auxiliary air supply means including means to provide auxiliary air from a point above said open vertical face downwardly to substantially all parts of said open face in a substantially uniform laminar flow: wherein said auxiliary air supply means comprises an entry chamber, a slot in one wall of said entry chamber having a width no more than one-half the dimension of said entry chamber parallel to said slot, and an expansion chamber extending from said slot and having an expansion angle of about 7l2, an air balance chamber, and means for air jet entrainment elimination.

18. A laboratory exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising:

a. a horizontal work surface, a rear wall, side walls,

and a front wall including an open vertical face immediately in front of said work surface; and

b. auxiliary air supply means including means to provide auxiliary air from a point above said open vertical face downwardly to substantially all parts of said open face in a substantially uniform laminar flow: wherein a bottom air foil is provided in front of and immediately above said horizontal work surface to define the bottom edge of said open face and air conduit means are provided between said auxiliary air supply means and said airfoil, including means to entrain room air across said work surface under said airfoil.

19. A laboratory exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising:

a. a horizontal work surface, a rear wall, side walls,

and a front wall including an open vertical face immediately in front of said work surface;

b. a sight-tight bypass within said front wall immedi- 5 ately above said open face;

c. a movable, vertical closure sash located on tracks immediately inside said front wall and operable to open such face and to close said bypass when at the upper end of said tracks and to close said face and to open said bypass when at the lower end; and

d. auxiliary air supply means including means to provide auxiliary air substantially vertically downwardly at a point above said open vertical face and in front of said bypass in substantially uniform laminar flow; wherein said auxiliary air supply means comprises an entry chamber, a slot in one wall of said entry chamber having a width no more than one-half the dimension of said entry chamber parallel to said slot, and an expansion chamber extending from said slot.

20. A laboratory exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising:

a. a horizontal work surface, a rear wall, side walls,

and a front wall including an open vertical face immediately in front of said work surface;

b. a sight-tight bypass within said front wall immediately above said open face;

c. a movable, vertical closure sash located on tracks immediately inside said front wall and operable to open such face and to close said bypass when at the upper end of said tracks and to close said face and to open said bypass when at the lower end; and

d. auxiliary air supply means including means to provide auxiliary air substantially vertically downwardly at a point above said open vertical face and in front of said bypass in substantially uniform laminar flow: wherein said auxiliary air supply means comprises: exterior vertical front, rear and end walls, a cover and interior baffles defining (I) an air entry chamber, (2) a slot at the top of the plenum and said entry chamber, (3) an expansion chamber extending vertically downwardly from said slot toward the interior of the plenum, and (4) an air supply outlet; wherein said chambers, slot and outlet extend substantially the length of said plenum, and said outlet extends substantially across the width of said plenum.

21. A laboratory exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising:

a. a horizontal work surface, a rear wall, side walls,

and a front wall including an open vertical face immediately in front of said work surface;

b. a sight-tight bypass within said front wall immediately above said open face;

0. a movable, vertical closure sash located on tracks immediately inside said front wall and operable to open such face and to close said bypass when at the upper end of said tracks and to close said face and to open said bypass when at the lower end; and

d. auxiliary air supply means including means to provide auxiliary air substantially vertically downwardly at a point above said open vertical face and in front of said bypass in substantially uniform laminar flow:

wherein a bottom air foil is provided in front of and immediately above said horizontal work surface to define the bottom edge of said open face, and air conduit means are provided between said auxiliary air supply means and said airfoil, including means to entrain room air across said work surface under said airfoil.

22. A laboratory exhaust hood including in combination a hood superstructure and an air supply plenum, comprising:

a. a horizontal work surface, a rear wall, side walls,

and a front wall including an open vertical face immediately in front of said work surface;

b. a sight-tight bypass within said front wall immediately above said open face;

c. a movable, vertical closure sash located on tracks immediately inside said front wall and operable to open such face and to close said bypass when at the upper end of said tracks and to close said face and to open said bypass when at the lower end; and d. auxiliary air supply means including means to provide auxiliary air substantially vertically downwardly through an outlet at a point above said open vertical face and in front of said bypass in substantially uniform laminar flow; said auxiliary air supply means also including anair balance chamber provided with a perforated back pressure plate and a shallow chamber between said air balance chamber and said outlet; wherein said back pressure plate is a fin-tube heat exchanger, whereby auxiliary air may be heated between said air balance chamber and said shallow chamber without affecting the substantially uniform laminar flow of air from said air supply outlet.

* i t i 

1. Apparatus for converting a rapidly moving air stream to slower air having substantially laminar flow free of localized non-uniform energy comprising: receiving means comprising an entry chamber for receiving said air stream; means including two diverging walls for expanding the flow of said stream substantially uniformly across the cross-section of its flow; and means for distributing the flow of said stream substantially uniformly from said receiving means to said expansion means; said distributing means comprising an elongated slot having a crosssectional area which is sufficiently smaller than the crosssectional area of said entry chamber transverse to the flow of said air to promote a spreading of the flow of said air into such slot along its length.
 2. Apparatus for converting a rapidly moving air stream to slower air having substantially laminar flow free of localized non-uniform energy, comprising: receiving means comprising an entry chamber for receiving said air streams; means including two diverging walls for expanding the flow of said stream substantially uniformly across the cross-section of its flow; and means comprising a passage between said receiving means and said expansion means for distributing the flow of said stream substantially uniformly from said receiving means to said expansion means, wherein said entry chamber and air expansion means are adjacent, share one of said walls as a common wall, and are constructed and arranged to reverse the direction of flow of said stream.
 3. Apparatus for converting a rapidly moving air stream to slower moving air having substantially laminar free flow of localized non-uniform energy, comprising means for receiving said air stream; means including two diverging walls for expanding flow of said stream substantially uniformly across the cross section of its flow; means comprising a passage between said receiving means and said expansion means for distributing the flow of said stream substantially uniformly from said receiving means to said expansion means; and means for air jet entrainment being downstream of said expansion means and including a multiplicity of closely spaced widely distributed air flow openings, together with means operatively associated with said openings upstream thereof for effecting a slight back-pressure.
 4. The apparatus of claim 3 further characterized by an air balance chamber between said air expansion means and said air jet entrainment elimination means.
 5. An auxiliary air supply plenum of modular design suitable for incorporation with a conventional laboratory exhaust hood, which hood is provided with an air exhaust means, an interior work space, a front face, and a movable closure sash in said face for supplying a substantially uniform laminar flow of auxiliary air to said exhaust hood, said auxiliary air supply comprising: exterior front, rear and end walls, a cover and interior baffles defining (1) an air entry chamber, (2) a slot along the upper edge of one wall of said entry chamber, (3) an expansion chamber extending vertically downwardly from said slot toward the interior of the plenum and (4) an air supply outlet downstream of said expansion chamber; wherein said chambers, slot and outlet extend substantially the length of said plenum and said outlet extends substantially across the width of said plenum.
 6. The air supply plenum of claim 5 wherein the width of said slot is no more than one-half the height of said entry chamber, and said entry chamber and said expansion chamber share a common wall havIng said slot at the top thereof.
 7. The air supply plenum of claim 5, wherein an air balance chamber and final air balance means are provided between said expansion chamber and said outlet, said air balance means including (1) a back pressure plate perforated with laminar flow holes substantially uniformly across its width and length, (2) an air jet entrainment eliminator, and (3) a shallow chamber between said plate and said entrainment eliminator.
 8. The air supply plenum of claim 7, wherein the dimensions and orientation of the elements thereof are such that: kAfw s + 2(l+h) tan ( theta /2) ; wherein k is at least 1.4, Af is the free area of said plate, w is the width of said plenum, s, the width of said slot, is 1.4-1.8 inches, l is the length of said expansion chamber, h is the height of said balance chamber, and theta , the expansion angle of said expansion chamber, is 7*-12*.
 9. The air supply plenum of claim 7, wherein said air supply outlet is provided on each of its end walls with curved profile vanes, whereby air exiting said plenum tends to assume a flow rate profile parabolic in shape along the length of the outlet, and the portion of the front wall defining the front of the air supply outlet is an adjustable vane hinged at the top and adjustable from the vertical toward the rear wall of the outlet, whereby the auxiliary air velocity may be varied.
 10. The air supply plenum of claim 7 wherein said back pressure plate is a fin-tube heat exchanger, whereby auxiliary air may be heated between said air balance chamber and said shallow chamber without affecting the substantially uniform laminar flow of air from said air supply outlet.
 11. A laboratory exhaust hood, having an open face, a bypass incorporated in said hood above said open face, a movable closure sash adapted to move from an open position, in which it closes said bypass and opens said face, to a closed position in which it opens said bypass and closes said open face, and means to maintain the total volumetric flow of air into said hood substantially constant during movement of said movable closure sash from its open position to its closed position comprising a plurality of substantially parallel walls arranged in said bypass sequentially in angular relation to provide a sight-tight labyrinth, wherein the angular relation of said walls forming said labyrinth to change direction within the range of substantially 50* to 70* at any one corner.
 12. The laboratory exhaust hood of claim 11, wherein said hood has a front wall in which said open face is located, and said hood is also provided with an auxiliary air supply plenum located above said open face, and wherein said bypass is constructed of a series of angled louver vanes extending horizontally across the length of said front wall from the bottom of said plenum to the top of said open face.
 13. The laboratory exhaust hood bypass of claim 12, wherein said louver vanes are angled to form a vertical series of V-shaped passages and adapted to induce an upward direction upon air traversing said bypass.
 14. A laboratory exhaust hood including in combination a hood superstructure having a vertical, rectangular face and a movable vertical closure sash, and an auxiliary air supply plenum, comprising: means within said auxiliary air supply plenum to provide auxiliary air substantially vertically downwardly from said plenum toward said open face of said hood superstructure and in substantially uniform laminar flow; bypass means operative upon the closing of said movable, vertical closure sash to direct at least a portion of said auxiliary air into said hood super-structure by passage through a sight-tight by-pass above said sash; and means to maintain the total volumetric flow of air into said hood superstructure substantially constant during movement of saId movable closure between its open and closed positions.
 15. The laboratory exhaust hood combination of claim 14, wherein further means are provided to control the maximum face velocity of the air entering the hood superstructure through said rectangular open face during the movement of said movable closure from its open position to its closed position between 2-3 times that of normal operation, with a fully opened sash.
 16. A laboratory exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising: a. a horizontal work surface, a rear wall, side walls, and a front wall including an open vertical face immediately in front of said work surface; and b. auxiliary air supply means including means to provide auxiliary air from a point above said open vertical face downwardly to substantially all parts of said open face in a substantially uniform laminar flow: wherein said auxiliary air supply means comprises an entry chamber, a slot in one wall of said entry chamber having a width no more than one-half the dimension of said entry chamber parallel to said slot, and an expansion chamber extending from said slot.
 17. A laboratory exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising: a. a horizontal work surface, a rear wall, side walls, and a front wall including an open vertical face immediately in front of said work surface; and b. auxiliary air supply means including means to provide auxiliary air from a point above said open vertical face downwardly to substantially all parts of said open face in a substantially uniform laminar flow: wherein said auxiliary air supply means comprises an entry chamber, a slot in one wall of said entry chamber having a width no more than one-half the dimension of said entry chamber parallel to said slot, and an expansion chamber extending from said slot and having an expansion angle of about 7*-12*, an air balance chamber, and means for air jet entrainment elimination.
 18. A laboratory exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising: a. a horizontal work surface, a rear wall, side walls, and a front wall including an open vertical face immediately in front of said work surface; and b. auxiliary air supply means including means to provide auxiliary air from a point above said open vertical face downwardly to substantially all parts of said open face in a substantially uniform laminar flow: wherein a bottom air foil is provided in front of and immediately above said horizontal work surface to define the bottom edge of said open face and air conduit means are provided between said auxiliary air supply means and said airfoil, including means to entrain room air across said work surface under said airfoil.
 19. A laboratory exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising: a. a horizontal work surface, a rear wall, side walls, and a front wall including an open vertical face immediately in front of said work surface; b. a sight-tight bypass within said front wall immediately above said open face; c. a movable, vertical closure sash located on tracks immediately inside said front wall and operable to open such face and to close said bypass when at the upper end of said tracks and to close said face and to open said bypass when at the lower end; and d. auxiliary air supply means including means to provide auxiliary air substantially vertically downwardly at a point above said open vertical face and in front of said bypass in substantially uniform laminar flow; wherein said auxiliary air supply means comprises an entry chamber, a slot in one wall of said entry chamber having a width no more than one-half the dimension of said entry chamber parallel to said slot, and an expansion chamber extending from said slot.
 20. A laboratorY exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising: a. a horizontal work surface, a rear wall, side walls, and a front wall including an open vertical face immediately in front of said work surface; b. a sight-tight bypass within said front wall immediately above said open face; c. a movable, vertical closure sash located on tracks immediately inside said front wall and operable to open such face and to close said bypass when at the upper end of said tracks and to close said face and to open said bypass when at the lower end; and d. auxiliary air supply means including means to provide auxiliary air substantially vertically downwardly at a point above said open vertical face and in front of said bypass in substantially uniform laminar flow: wherein said auxiliary air supply means comprises: exterior vertical front, rear and end walls, a cover and interior baffles defining (1) an air entry chamber, (2) a slot at the top of the plenum and said entry chamber, (3) an expansion chamber extending vertically downwardly from said slot toward the interior of the plenum, and (4) an air supply outlet; wherein said chambers, slot and outlet extend substantially the length of said plenum, and said outlet extends substantially across the width of said plenum.
 21. A laboratory exhaust hood including in combination a hood superstructure and an auxiliary air supply plenum, comprising: a. a horizontal work surface, a rear wall, side walls, and a front wall including an open vertical face immediately in front of said work surface; b. a sight-tight bypass within said front wall immediately above said open face; c. a movable, vertical closure sash located on tracks immediately inside said front wall and operable to open such face and to close said bypass when at the upper end of said tracks and to close said face and to open said bypass when at the lower end; and d. auxiliary air supply means including means to provide auxiliary air substantially vertically downwardly at a point above said open vertical face and in front of said bypass in substantially uniform laminar flow: wherein a bottom air foil is provided in front of and immediately above said horizontal work surface to define the bottom edge of said open face, and air conduit means are provided between said auxiliary air supply means and said airfoil, including means to entrain room air across said work surface under said airfoil.
 22. A laboratory exhaust hood including in combination a hood superstructure and an air supply plenum, comprising: a. a horizontal work surface, a rear wall, side walls, and a front wall including an open vertical face immediately in front of said work surface; b. a sight-tight bypass within said front wall immediately above said open face; c. a movable, vertical closure sash located on tracks immediately inside said front wall and operable to open such face and to close said bypass when at the upper end of said tracks and to close said face and to open said bypass when at the lower end; and d. auxiliary air supply means including means to provide auxiliary air substantially vertically downwardly through an outlet at a point above said open vertical face and in front of said bypass in substantially uniform laminar flow; said auxiliary air supply means also including an air balance chamber provided with a perforated back pressure plate and a shallow chamber between said air balance chamber and said outlet; wherein said back pressure plate is a fin-tube heat exchanger, whereby auxiliary air may be heated between said air balance chamber and said shallow chamber without affecting the substantially uniform laminar flow of air from said air supply outlet. 